Delta modulation is commonly employed in communication systems such as telephone systems to convert the rate of change (slope) of an analog input signal, i.e., voice signal, into a binary bit stream for transmission. The received binary bit stream is then employed to reconstruct the analog input signal. Delta modulators are conventional and, while quite simple, suffer from an inability to respond to sudden changes in signal amplitude. This problem, commonly termed "slope overload," requires some type of solution which, unfortunately, then produces another disadvantage which is the generation of excessive "quantizing noise" for low level signals. Prior art approaches to the problem include the use of adaptive schemes wherein special circuitry is employed to adjust the size of the quantizing steps in the reconstructed signal in order to more nearly approximate the original signal.
Adaptive systems themselves have certain disadvantages in addition to the electronic complexity thereof and one of these disadvantages arises from the length of time required for the adaptive circuits of the demodulator to lock into the proper reference level established by the modulator. This "initializing time" is often long compared to the period of speech waveforms with the result that noise is generated by the demodulator until it has arrived at the proper reference level. Another disadvantage of adaptive systems arises when there are abrupt transitions from speech signals to silence, inasmuch as a brief noise pattern is produced which may persist at a level which is dependent on the nature of the transition and may be a source of annoyance to a listener because of signal dependence thereof. Additionally, adaptive systems normally employ one or more closed feedback loops in the modulator and it is recognized that such loops are potentially unstable and have a tendency to oscillate, which may cause the reference levels to fluctuate and thus cause the demodulator to follow these fluctuations with the general result of generating a noise at the frequency of the loop oscillation.
The present invention provides the advantages of adaptive systems without incorporating the above-noted disadvantages.